It is known in the art that small compact acceleration sensors may be formed by micromachining silicon wafers into suitable configurations that are capable of detecting acceleration along one axis. The micromachining process is normally performed on batches of silicon wafers. This process consists of masking and forming patterns of etch stop material on a wafer surface, etching the exposed silicon, removing the etch stop material, metallizing, and bonding. The silicon wafers are diced into individual acceleration sensor devices which are packaged and connected to suitable electronic circuitry to form accelerometers. Using these techniques, a two axis or three axis acceleration sensor requires two or three discrete diced devices, respectively, to be precisely mechanically aligned along two or three orthogonal axes of acceleration. Examples of acceleration sensors formed by a micromachining process are described in the following U.S. Pat. Nos. 4,574,327; 4,930,043; and 5,008,774.
Prior forms of silicon acceleration sensors employ an inertial mass which moves in response to acceleration, positioned by cantilever support members that may introduce an asymmetry that can result in an undesirable cross-axis sensitivity. To avoid this undesirable asymmetric effect, these devices are designed with flexible support members around the periphery of an inertial mass so that the response to acceleration is preferentially along an axis perpendicular to the plane of the inertial mass and the support members. To further limit the acceleration response to one axis, the support members are sometimes placed in the mid-plane of the inertial mass or symmetrically placed at the top and bottom surfaces of the inertial mass. The devices fabricated in this manner may exhibit wide parameter variations between devices. Furthermore, for multiple axes applications, multiple discrete devices must be precisely aligned mechanically to each axis of acceleration. Difficulties encountered in the fabrication include the accurate location of the mid-plane and precise alignment of multiple devices, making the fabrication process complex, slow and expensive.
For the foregoing reasons, there is a need for a monolithic multiple axes acceleration sensor micromachined from silicon by a relatively simple fabrication process that results in low mechanical stress, temperature stable devices with tight parameter tolerances between devices. It is desirable that any required multiple axes alignment be performed as a part of the lithographic process used in the device fabrication rather than require precise mechanical alignment of discrete devices after the dicing operation. It is further desirable that the fabrication process be adjustable on a batch basis, in order to produce devices with predetermined acceleration sensitivity, with batches ranging from low sensitivity devices to high sensitivity devices.